It is known to compression encode source information in order to reduce an amount of data which the source information represents. A well known example of this is the “ZIP” coding application on conventional personal computers which allows data files to be compressed so that the information of the data file car be represented as a much smaller amount of data. This allows the information represented in the data file to be communicated by for example electronic mail, or stored on a smaller capacity disc or a smaller number of discs. Another example of data compression encoding is the encoding applied to data representative of video and audio signals, such as, for example, the standard ISO/IEC/13818-2, known as MPEG2 [4].
Although the amount of data produced after the source information has been compression encoded will of course vary in accordance with the amount of data representing the original source information, the MPEG2 encoding algorithm is one example of a compression encoding algorithm in which the amount of data produced after the source information is compression encoded varies in dependence upon the content of the source information. As a result an information source generating data at a substantially constant source data rate will result in a compression encoded data having a data rate which varies.